Haemophilus parasuis is a γ-proteobacteria and is classified in the Pasteurellaceae family. H. parasuis is a rod-shaped, non-haemolytic, gram-negative nicotinamide adenine dinucleotide (NAD)-dependent bacterium (Biberstenia and White, 1969; Nicolet 1992). H. parasuis is a commensal of the upper respiratory tract of healthy pigs. It is also an important pathogen and the etiological agent of Glasser's disease, which is typically characterized by fibrinous polyserositis, polyarthritis, meningitis and sometimes acute pneumonia and septicemia (Amano et al., 1994; Peet et al., 1983; Little, 1970). This disease is one of the main causes of swine mortality in the US and swine industries worldwide, resulting in huge economic losses.
Recent studies have demonstrated that non-virulent strains are more prevalent in the upper respiratory tract than virulent strains however, they are not protected against systemic infection by virulent strains.
Based on heat-stable antigens and gel diffusion test data, fifteen serovars of H. parasuis have been identified so far, but a high percentage of the field isolates are non-typable (Kielstein and Rapp-Gabrielson, 1992). Different serovars of H. parsuis exhibit different virulence, ranging from highly virulent to nonvirulent. Of the 15 recognized serovars, serovar 5 strains are isolated more frequently worldwide than any other. The serovar 5 strain is also frequently isolated from respiratory and systemic infection in pigs and is highly virulent (Rapp-Gabrielson and Gabrielson, 1992; Blackall et al., 1996; Oliveira, 2003). Furthermore, it has emerged recently as one of the major causes of neonatal mortality in the pig industry worldwide as well.
Newport Laboratories developed a commercial vaccine (ParaSail) against H. parasuis infection using a highly virulent serovar 5 North American field isolate. This vaccine—when used as recommended by swine producers—is highly effective in mitigating H. parasuis infection in the United States. Recently, however, a few farms in the Midwestern United States using this vaccine had reported outbreaks of severe pneumonia. Tissue samples obtained by Newport Laboratories during the 2011-2012 outbreak yielded a highly virulent isolate of H. parasuis serotype 4 strains. The reason for outbreak was unknown at the time, although internal challenge studies showed that ParaSail vaccine when used at 109 CFU per dose protects pigs against H. parasuis serotype 4 strains.
Like the rest of the members of Pasteurellaceae, H. parasuis has a large number of virulence and virulence-associated genes. These include lipopolysaccharide (LPS), capsular polysaccharide, adhesins/fimbriae, outer membrane proteins, neuraminidase, iron and heavy metal acquisition and transport systems (Amano et al., 1994; Biberstein, 1990; Munch et al., 1992; Zucker et al., 1996; Morozumi and Nicolet, 1986). The molecular basis underlying these candidate virulence factors is yet to be fully elucidated, however, due to the complex gene regulatory mechanisms involved. H. parasuis strains exhibit high heterogeneity at the molecular level, mostly due to recombination or lateral gene transfer. In order to elucidate the possible reasons behind the 2011-2012 Midwest outbreaks at the molecular level we sequenced the genomes of two H. parasuis serotype 4 isolates and performed a 3-way comparative genomic analysis against our Parasail vaccine strain (used as a reference; SEQ ID NO. 1). The two H. parasuis serotype 4 isolates were obtained from Newport Laboratories case numbers 12-1322 (SEQ ID NO. 2; GenBank accession number JJNQ00000000; ST4-1) and 11-1398-2 (SEQ ID NO. 3; GenBank accession number JJNR00000000; ST4-2).
Although neither the mechanisms involved in the protective immunity following controlled exposure nor the identity of antigens which provide protective immunity are clear at this time, it is known that protective immunity can be induced by such exposure.
The pork industry is in immediate need of a safe and efficacious product capable of aiding the prevention of disease, specifically Haemophilus parasuis serotype 4.